Speed control in induction motor drive systems, especially asynchronous motors, can be effected using semiconductors with certain firing characteristics. The semiconductor systems can generate a variable frequency from the alternating current network or line frequency through the intermediary of a direct-current circuit in so-called frequency-converter systems. Direct-current sources, such as batteries, may also be used as inputs for the semiconductor elements which generate the speed-control frequency of the motor.
The principle of frequency-converter motor control recognizes that with an increasing frequency there is an increasing voltage at the motor of the frequency converter system.
Losses in such systems between the network or line current source and the motor generally are a function of the magnitude of the current and can result in a motor efficiency which at low speeds can be less than 50%.
The losses can result, inter alia, from a transformation of electrical energy into mechanical energy in the motor, from losses within the motor which are current dependent and from losses in the frequency converter and the circuitry supplying the motor.
Generally the motor must operate at variable speeds including, for example, a low speed for washing which can apply for a period which is significantly longer than the relatively short time required for centrifugation in a washing machine wherein the motor must operate at high speed.
Similar speed differences are encountered in electric vehicles which must be capable of being driven at high speed and low speed.
As a consequence of losses in earlier drive systems involving asynchronous motors and converters, and as a result of the need to enable switching between higher and lower speeds and vise versa, the motors used tend to be relatively large, requiring converters of substantial size and involving high energy losses and costs.
Another drawback of earlier systems is the fact that the earlier systems produce substantial magnetic noise to the detriment of electronic equipment in the vicinity.